The present invention relates to a method and apparatus for mounting a femoral stem hip prosthesis in a femoral canal with the use of a cement. It is well known in the prior art to initiate total hip replacement in individuals where: (1) a painful and/or severely disabled joint results from osteo arthritis, traumatic arthritis, or rheumatoid arthritis; (2) there is an occurrence of persistent or recurrent pain and/or physical impairment subsequent to femoral head replacement, cup arthroplasty, or other conventional techniques; (3) difficult clinical management problems occur where experience has indicated that more conventional arthroplasty techniques are not likely to achieve satisfactory results or where arthrodesis is contra-indicated because of age, sex, occupation, or height of the patient, or (4) bone stock is of poor quality of inadequate for other reconstruction techniques as indicated by deficiencies of the femoral head, neck or acetabulum. The femoral stem hip prosthesis is not confined to the replacement of total hips. Some patients need only the femoral head and neck to be replaced. In these cases, the head of the femoral prosthesis is inserted into the patient's existing acetabulum. In this type of prosthesis, the stem may or may not be cemented in. The most commonly used prosthesis of this type are the Thompson and Moore unfenestrated prostheses.
Where total hip replacement is required, the patient's hip socket, the acetabulum, and the femur just be prepared to receive mating components. The acetabulum is modified to receive an acetabular cup which mates with the head of the femoral stem prosthesis which is attached to the femur.
In order to prepare the femur for the insertion of the femoral stem prosthesis, the proximal end of the femur is prepared by retracting the muscle at the greater trochanter laterally and freeing any soft tissue which might be present. After the proximal end of the femur is exposed, the head and neck is then resected with a reciprocating saw in a 5.degree. to 10.degree. anteverted plane or as medically indicated and obliquely from the base of the neck laterally to the base of the head medially so that the angle of the cut approximates the angle of the prosthetic platform. The femur is then prepared by rasping or curetting the intramedullary canal in its proximal end and extending this process well laterally and distally in the proximal shaft to accommodate a cement mass for support and fixation of a stem of a femoral prosthesis. Once the canal has been properly prepared by reaming and curettage, the distal canal is plugged, utilizing a bone plug obtained from the femoral head. An alternate method of plugging the canal utilizes a bolus of acrylic cement in the "rubbery" elastic stage just prior to setting and tamping the plug down distally 2 cm. below the stem tip. In both cases, the plugs effectively seal off the canal to prevent excessive penetration of cement below the stem prosthesis tip. The femoral canal is then dried and the acrylic cement is mixed and poured into a syringe and immediately injected into the dried femoral canal from distal to proximal. A rubber dental dam may be stretched over the proximal femur and the acrylic digitally finger packed. After the acrylic has reached the dough stage, the dam may be removed and additional finger packing is vigorously performed for at least a minute to two minutes to insure adequate filling and penetration.
After the cement has been packed into the proximal femur, the femoral prosthesis is carefully inserted to avoid varus placement. When the acrylic has reached the late dough stage, the stem of the prosthesis is inserted into the femoral canal and either held by hand or by the use of an inserter; the prosthesis is carefully held in place during the final stages of setting or hardening of the acrylic. Present state of the art requires the femoral stem prosthesis to be held in its desired position manually for approximately six to twelve minutes while the cement hardens. Two types of holders or drivers presently used are Zimmers #4044-23 Driver or Zimmers 4046-10 Femoral Prosthesis Holder. Either of these two requires the operating surgeon to hold steady the femoral head with constant and non-pulsating pressure for six to twelve minutes. This is humanly impossible and when the cement has hardened, there are varying degrees of looseness of the cement about the prosthesis. When the patient begins walking and the prosthesis becomes weight bearing, it is possible that the femoral stem and cement become loose and may require re-cementing. In still other instances, the metal femoral stem of the prosthesis may break in the region of the middle third of the metal stem. This occurs because the distal third of the metal stem is held securely but the proximal position is loose and when weight is applied, the force is too great and the metal breaks.
Further, there has recently been introduced the Miller Cement Delivery System by Zimmer and the Exeter Pressurized System by Howmedica which temporarily pressurizes the cement into the lower part of the femoral bone canal by use of an inflated balloon or rubber plug. This pressure is released when the plug or balloon is removed so that the prosthesis can be inserted in the femoral canal. The proximal portion of the femoral bone canal is then packed with cement digitally by pushing the cement into the canal and about the prosthesis with the fingers. The problem, of course, is that the pressure is released when the prosthesis is inserted and, again, the prosthesis must be hand-held until the cement hardens.
Further, in the present state of the art, it is the practice of the surgeon to insert the femoral stem prosthesis into the femoral canal and observe or "eyeball" the overall anticipated position and placement of the prosthesis. He attempts to place the distal end of the femoral stem medially in the femoral canal rather than laterally. Finally, a decision is made to determine that the position is satisfactory and the surgeon proceeds to place the cement into the femoral canal and hold the prosthesis manually in the same previous "eyeballed" position until the cement hardens. At a later time, perhaps hours or days after the surgery, an X-ray is taken which may reveal the distal end to be more laterally positioned than was supposed at the trial insertion during surgery. The surgeon may not have recognized the slight shift of the stem of the prosthesis while inserting it into the cemented femoral canal. Further, the surgeon may only assume that the prosthesis collar abutts the femur calcar as he inserts the prosthesis into the cemented canal. Also, the failure to insert the prosthesis to exactly the same depth as during the trial position may occur.
Also, the positioning of the head and neck of the prosthesis in anteversion, a relationship of the head and neck of the prosthesis with the shaft of the femur, may not be ascertained accurately by "eyeballing". Thus, positioning of the prosthesis in varying degrees of valgus or varus is important and is related in part to the position of the distal end of the femoral stem. It is also related to the position of the proximal end of the prosthesis stem in the intertrochanteric area.
A variety of changes of position may occur from the "eyeballing" position to the cemented position but cannot be determined or fully evaluated until the cement has hardened. At that time, of course, the task of removing the prosthesis and cement is a major additional operation if undertaken.
Also, the surgeon must presently pack the cement beneath the collar of the prosthesis and about the bone calcar with the fingers, and it is nearly impossible to do it accurately with this method.
Finally, the bone reacts to the stress and strains that are placed upon it. Thus, it builds a stronger, thicker cortex where the stress is greater and weaker, thinner cortex where the stress is less. Anatomically, the cortex is thicker at the calcar and on down the anteromedial aspect of the shaft of the femur. This is so because this segment of bone bears most of the weight and stress in walking, running and climbing steps. Presently, it is a chore of varying skill on the part of the surgeon to cover well the cut surface of the calcar or the cut surface of the anteromedial aspect of the femur. When the cut surface of the cortex is not capped with cement, the resultant load on the femoral stem prosthesis is transmitted down the canal of the femur. When load pressure is applied to the prosthesis and cement, if the cortex is not capped with cement, the effect is to wedge the cement into the femoral canal. This occurs because the proximal end of the femoral bone canal is greater in circumference than is the bone canal at the junction of the proximal and middle third of the femoral bone canal. This wedged pressure exerts outward pressure on the cortex in addition to the longitudinal pressure from the weight of the individual. When the cut end of the femur is not capped with cement, the resultant forces are not natural or physiological and, in some instance, wasting away of the calcar and the anteromedial aspect of the femoral shaft is seen by X-rays months later due to atrophy or disuse.
Thus it is an object of the present invention to provide constant pressurization on the cement during the hardening of the cement and a fixed jig which holds the prosthesis in a fixed relationship to the femur, thus eliminating all motion of the prosthesis while the cement is hardening. Further, the constant pressurizing of the cement produces packed cement in the femoral canal and about the entire length of the prosthesis stem so that the pressure is applied higher on the metal stem and more evenly on the entire metal stem with a weight load pressure being applied on a much shorter lever arm and thus having less possibility of breaking the metal stem. Further, pressurizing the cement in the canal during hardening will decrease loosening of the cement-stem unit. Thus, the present invention obviates the strain placed upon the surgeon while holding the impacter, and thus the prosthesis, motionless for approximately six to twelve minutes.
It is still another object of the present invention to provide a device which allows constant pressurized cement to fill the femoral canal while the cement is hardening, thus securely engaging the steam prosthesis.
Further, the present invention allows the femoral stem prosthesis to be placed in position prior to applying cement and the position of the stem, the position of the collar on the calcar, the length of the neck, the anteversion and the varus or valgus to be determined by X-ray. If any of these positions are not satisfactory, the stem prosthesis can be repositioned to correct these factors and held rigidly in the new position while being X-rayed again to determine the final position in which the prosthesis will rest when secured with cement.
Also, the use of the constantly pressurized cement not only packs the cement under the collar of the prosthesis, but maintains pressure on the cement in this area as well as the entire cement mass while it hardens. Finally, the present invention which utilizes a constantly pressurized cement causes the calcar and approximately the medial 2/3rd of the cut surface of the femur to be capped with the cement.
This enables the weight load of the person to be transmitted to the cortex of the femur as well as the cement. The other forces of stress and strain and torgue are transmitted to and through the cement as nearly normal as presently possible to simulate the medullary trabeculae.
Some surgeons use a collarless femoral stem prosthesis and operate on the theory that it is not necessary to cover the cut surface of the femur with cement. This group of surgeons believes that if the cement is inserted well into the proximal femur that, with the patient's weight bearing, the femoral stem and cement will wedge tightly into the femoral canal if "settling" of the prosthesis occurs or if bone absorption occurs. A recent collarless femoral stem prosthesis is De Puy's Dual-Locking.